The perilymphatic perfusion with artificial perilymph containing sodium bromate (lmM) was employed in guinea pigs to study the ototoxity of bromate. Changes in the cochlear potentials and electrolyte concentrations in the endolymph were measured in both control and experimental perfusions. The perilymphatic application of bromate resulted in a gradual suppression of the endocochlear potential and sound evoked responses. The recovery of the cochlear potentials was not observed. A substantial decrease in potassium and chloride concentrations and increase in sodium concentration were observed in the endolymph. Our results indicated that the primary site of action of bromate was on the stria vascularis and in advanced stage of intoxication bromate might affect the hair cells, causing a further suppression of the hair cell responses.